Its great to hear that its normal again, Brian! Its also interesting to see what different ideas people come up with as possible scenarios. INcluding myself.
Its also great when noises go away on their own.
I had a real noisy engine after getting my $450 '87 300D project running. I had put a lot of time and $$ in to it and after I discovered that the Trap Oxidizer (make that Nasty T.O.) was plugged I took it to the dealer for its trapectomy. All that time it ran strong but it had a loud nailing. I rechecked timing with the RIV tool and it was spot on. I swapped another set of injectors onto it but still the same darned nailing. It was very pronounced and bothered me a lot
having just installed all the new parts I could afford to make sure this was going to be a reliable engine: it had new rings, a rebuilt head, new lifters, a new vac pump, new radiator, NEW water pump, new oil cooler lines, new oil cooler because I stripped the threads trying to get the hose fittings off, etc. etc. then what a let down with all the clatter, I was really embarassed to start it up.
Then I started thinking and I remembered the car had no gas cap on it when I got it. It had been sitting out for almost 2 years. The PO had some kids around and they may have put water in the the fuel tank because when I pumped some fuel out into a small glass jar I could see it had a milky appearance.
I pumped most of the fuel into a 55 gal drum then filled the tank with a bottle of RedLine catalyst and fresh fuel. That helped some but it was still nailing. After I ran thru two tanks of fuel and redline treatment and some Diesel Power Kleen stuff (not sure if that's the proper name) the noise slowly dimished and its completely normal now. I'll never understand what was going on. Maybe the rings were seating in also.